1. Technical Field
Several aspects of the present invention relate to a solid electrolyte, a method for producing a solid electrolyte, and a lithium-ion battery.
2. Related Art
As a power source for many electrical apparatuses such as portable information apparatuses, a lithium battery (including a primary battery and a secondary battery) has been used. In particular, as a lithium battery having both of a high energy density and safety, an all-solid-state lithium battery using a solid electrolyte for lithium conduction between positive and negative electrodes has been proposed (see, for example, JP-A-2009-215130 (PTL 1)).
A solid electrolyte can conduct lithium ions without using an organic electrolytic solution, and does not cause leakage of an electrolytic solution or evaporation of an electrolytic solution by heat generation due to driving or the like, and therefore has been drawing attention as a material with high safety.
As such a solid electrolyte to be used in an all-solid-state lithium battery, an oxide-based solid electrolyte having a high lithium ionic conductivity, an excellent insulating property, and high chemical stability has been widely known. As such an oxide, a lithium lanthanum titanate-based material has an especially high lithium ionic conductivity, and therefore has been expected to be applied to batteries.
In the case where such a solid electrolyte is in the form of particles (hereinafter sometimes referred to as “solid electrolyte particles”), the solid electrolyte is often molded to conform to a desired shape by compression molding. However, the solid electrolyte particles are very hard, and therefore, in the resulting molded product, the contact of the solid electrolyte particles with one another is not sufficient to increase the grain boundary resistance, and thus, the lithium ionic conductivity tends to be decreased.
As a method for decreasing the grain boundary resistance, a method in which after solid electrolyte particles are compression-molded, the resulting molded body is sintered at a high temperature of 1000° C. or higher, whereby the particles are welded to one another is known. However, with this method, the composition is liable to be changed due to high temperature heat, and thus it is difficult to produce a solid electrolyte molded body having desired physical properties.
Therefore, as a method for decreasing the grain boundary resistance of a solid electrolyte, a method in which after the surface of each lithium lanthanum titanate particle is coated with SiO2, the particles are sintered at a high temperature has been studied (see, for example, JP-A-2011-529243 (PTL 2)).
On the other hand, as a method for forming a solid electrolyte, a synthesis system using a liquid phase material, particularly a sol-gel method is sometimes adopted. By the sol-gel method, for example, lithium lanthanum titanate can be produced (see, for example, JP-A-2003-346895 (PTL 3)).
However, the above methods have problems as follows. The method disclosed in PTL 2 has a problem that it is difficult to coat the surface of each solid electrolyte particle with SiO2. In addition, by performing firing at a high temperature, lithium is evaporated from the solid electrolyte to be obtained or reacts with a material constituting an electrode to change the composition, and moreover, a large amount of a different phase may be formed. When the firing temperature is decreased for preventing the formation of a different phase, the boundary surface between the particles is not sufficiently sintered, and therefore, the grain boundary resistance cannot be decreased.
The method disclosed in PTL 3 has a problem that since the product forms a uniform layer, and therefore, it is difficult to control the structure of the solid electrolyte particles to be formed, and thus, desired physical properties are hardly obtained.